One of the unique features of many metallocene catalysts is that they tend to produce polyolefins having a narrow molecular weight distribution. While the narrow molecular weight distribution has benefits in some applications, for other applications there is often a need for a polyolefin product having a broader molecular weight distribution. For example, while narrow molecular weight distribution polymers are often suitable for injection molding, and possibly for the production of fibers, other applications such as thermoforming, extrusion, blow molding, and the production of foams or film often are found to perform better with polymers having a broader molecular weight distribution.
In the past there have been some patents which suggest the employment of a mixture of metallocenes to produce polymers having broadened molecular weight distributions. Examples of those patents include U.S. Pat. No. 4,530,914 and 4,937,299. While these two patents at least suggest that a wide range of mixtures of metallocenes could be employed in producing the broadened molecular weight distribution, all examples in those patents involve the employment of unbridged metallocenes. The applicants' recent research has revealed that many unbridged metallocenes are particularly sensitive to the presence of hydrogen. Accordingly, some combinations of unbridged metallocenes while producing a broadened molecular distribution when used in the presence of hydrogen produce polymers having such low molecular weight that they would not be suitable for many applications.
An object of the present invention is to provide a mixed metallocene catalyst system which is capable of giving broad molecular weight distributions over a wide range of polymerization conditions, including polymerizations employing a wide range of hydrogen levels. Particularly preferred embodiments produce polymer products having a molecular weight distribution greater than 8, even more preferably greater than 30.
In accordance with another aspect of the present invention, there are provided catalyst systems capable of producing unusual copolymers in which the comonomer incorporation is incorporated mainly in the high molecular weight end of the molecular weight distribution.
In accordance with yet another aspect of the present invention, there are provided catalyst systems capable of providing ethylene copolymers of broad molecular weight distribution which have high environmental stress crack resistance.